1. Field of the Invention
The present invention relates to a multilayer wiring board that includes a plurality of electrodes for external connection, the electrodes being disposed on one principal surface of the multilayer wiring board, and a surface insulating film configured to prevent a short-circuit between adjacent electrodes for external connection.
2. Description of the Related Art
With downsizing of electronic devices, a multilayer wiring board that allows a three-dimensional wiring structure has become widely used as a wiring board for forming an electric circuit for modules mounted on a motherboard of an electronic device (see, e.g., paragraphs [0035] to [0049] and FIG. 1 in Japanese Unexamined Patent Application Publication No. 2013-38385). As illustrated in FIG. 8, a multilayer wiring board 100 of this type is formed by a ceramic multilayer body 102 which is obtained by stacking a plurality of ceramic sheets 101. Each of the ceramic sheets 101 is provided with various wiring electrodes 103a and 103b on one principal surface thereof, and interlayer connection conductors 105 passing through the ceramic sheet 101 in the thickness direction. The wiring electrodes 103a and 103b between layers are connected to each other by the interlayer connection conductors 105.
With further downsizing of modules in recent years, there has been a demand for a reduction in distance between adjacent ones of the wiring electrodes 103a and 103b and between the wiring electrode 103a or 103b and the interlayer connection conductor 105 adjacent to each other. In the multilayer wiring board 100, the outer edge of each of the wiring electrodes 103a and 103b is covered with an insulation covering film 104 to prevent a short-circuit between the wiring electrodes 103a and 103b which are not originally designed to be connected and between the wiring electrode 103a or 103b and the interlayer connection conductor 105.
Each wiring electrode 103a on the upper surface of the multilayer wiring board 100 is used, for example, as an external connection electrode for connection to an external motherboard. In this case, part of the wiring electrode 103a may be used as a ground electrode. For better grounding performance, it is preferable to increase the area of the ground electrode. As the area of the ground electrode increases, however, it becomes difficult to determine an appropriate amount of solder for connection to the motherboard. This results in an increased risk of mounting failure during connection to the motherboard. Therefore, in the related art, a plurality of ground electrodes are formed to ensure mountability of the multilayer wiring board while maintaining the total area of ground electrodes.
The ground electrode described above may be used as a shielding electrode to prevent electromagnetic coupling with the motherboard. However, when a plurality of ground electrodes are formed, a space where there is no electrode is created between ground electrodes. Therefore, to prevent the electromagnetic coupling described above, it is necessary to provide a large-area ground electrode on another ceramic sheet 101. This leads to an increased number of ceramic sheets 101 and makes it difficult to reduce the thickness of the multilayer wiring board 100.